Mechanisms and Longevity of Strain Localization during Dynamic Recrystallization of Olivine

橄榄石动态再结晶过程中应变局域化的机制和寿命

• 批准号: 1249737
• 负责人: Whitney Behr
• 金额: $ 22.81万
• 依托单位: University of Texas at Austin
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2013
• 资助国家: 美国
• 起止时间: 2013-09-01 至 2019-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1249737&HistoricalAwards=false
• 关键词: Mechanisms; Longevity; Strain; Localization; during

This project investigates the role of grain size reduction by dynamic recrystallization in promoting strain localization in dry olivine aggregates using a suite of laboratory experiments coupled with detailed microstructural analyses. There are two primary goals of the project. The first is to identify the ways in which dynamic recrystallization can lead to localization under different conditions of stress, temperature, grain size, and strain magnitude. The processes that are most commonly proposed in the literature include a) geometric softening, b) a switch to grain size- sensitive deformation mechanisms, either diffusion creep or grain boundary sliding, and c) recovery by grain boundary migration. One or all of these mechanisms may contribute to the pronounced weakening observed in olivine under specific experimental conditions and in nature, and each should exhibit unique mechanical and microstructural signatures. The investigators will integrate detailed microstructural and mechanical observations over a range of experimental conditions, to systematically determine which of these mechanisms are dominant. The second goal is to examine the conditions under which dynamic recrystallization will result in permanent, as opposed to transient localization, by evaluating the role of syn-deformational grain growth. The project uses several unique ways of quantifying the role of grain growth in dynamically recrystallizing olivine aggregates, and of distinguishing between surface energy and strain energy driven grain boundary migration for a range of experimental conditions. The results of this research will have several important implications for geodynamics and mantle rheology, including the following. 1) The mechanical data will improve current deformation mechanism maps for dry olivine and can be incorporated into both large-scale mantle convection models and smaller-scale models of transient instabilities and mantle seismicity. 2) The processes identified as contributing to the development of olivine lattice preferred orientation can be input into models of olivine olivine lattice preferred orientation evolution and associated seismic anisotropy. 3) Distinguishing the conditions under which grain growth will counteract grain size reduction will allow evaluation of theoretical descriptions of the relationship between stress and grain size (the piezometric relationship). 4) The identification of microstructural criteria that are diagnostic of specific localization processes can be extrapolated to naturally deformed rocks and used to infer localization mechanisms and associated mechanical behavior of rocks under natural conditions.Understanding why deformation in Earth's rigid outer shell, the lithosphere, is commonly localized into faults and shear zones, rather than distributed over wide distances, is a fundamental question in geodynamics. These localized faults and shear zones are unique to planet Earth and are the reason parts of Earth exhibit rigid, plate-like behavior, in contrast to the more distributed deformation typically observed on other planets. Decades of observation of faults and shear zones where they cut the crust and upper mantle reveals that they are almost always associated with a significant reduction in grain size, which suggests that grain size reduction may be one of the most efficient mechanisms of localizing deformation. This project investigates mechanisms of grain size reduction in mantle rocks. Specific questions to be addressed include: under what conditions does grain size reduction lead to localization in mantle rocks? By what processes does the grain size reduction cause localization? And how long will the localization last on geological timescales? These questions will be addressed through integrated rock deformation experiments on olivine as well as detailed microstructural analysis of the experimentally deformed products.

本项目研究的作用，通过动态重结晶，促进应变局部化的干燥橄榄石聚集体的晶粒尺寸减小使用一套实验室实验，再加上详细的显微结构分析。该项目有两个主要目标。首先是确定在不同的应力、温度、晶粒尺寸和应变量条件下动态再结晶导致局部化的方式。在文献中最常提出的过程包括a）几何软化，B）切换到晶粒尺寸敏感的变形机制，扩散蠕变或晶界滑动，以及c）通过晶界迁移恢复。这些机制中的一个或全部可能有助于在特定的实验条件下和在自然界中观察到的橄榄石中的显著弱化，并且每个机制都应该表现出独特的机械和微观结构特征。研究人员将在一系列实验条件下整合详细的微观结构和力学观察，以系统地确定这些机制中的哪一个占主导地位。第二个目标是检查的条件下，动态再结晶将导致永久的，而不是瞬时本地化，通过评估同变形晶粒生长的作用。该项目使用了几种独特的方法来量化动态再结晶橄榄石聚集体中晶粒生长的作用，并区分表面能和应变能驱动的晶界迁移的实验条件范围。这项研究的结果将对地球动力学和地幔流变学有几个重要的影响，包括以下几点。1)力学数据将改善目前的变形机制地图干橄榄石，可以纳入大规模的地幔对流模型和较小规模的模型的瞬态不稳定性和地幔地震活动。2)被确定为有助于橄榄石晶格优选取向的发展的过程可以被输入到橄榄石橄榄石晶格优选取向演化和相关的地震各向异性的模型中。3)区分晶粒生长将抵消晶粒尺寸减小的条件将允许评估应力和晶粒尺寸之间的关系（测压关系）的理论描述。4)诊断特定局部化过程的微观结构标准的识别可以外推到自然变形的岩石，并用于推断自然条件下岩石的局部化机制和相关力学行为。了解为什么地球刚性外壳（岩石圈）的变形通常局限于断层和剪切带，而不是分布在很远的距离上，是地球动力学的一个基本问题。这些局部断层和剪切带是地球所独有的，也是地球部分地区表现出刚性、板块状行为的原因，与其他行星上通常观察到的更分散的变形形成对比。对断层和剪切带切割地壳和上地幔的数十年观察表明，它们几乎总是与粒度的显著减小有关，这表明粒度减小可能是局部变形的最有效机制之一。本计画研究地幔岩石中的晶粒缩小机制。要解决的具体问题包括：在什么条件下，晶粒尺寸减小导致地幔岩石中的本地化？晶粒尺寸的减小通过什么过程导致局部化？在地质时间尺度上，定位会持续多久？这些问题将通过对橄榄石的综合岩石变形实验以及对实验变形产物的详细显微结构分析来解决。